Hydrogenated sugar (also referred to as “sugar alcohol”) means a compound obtained by adding hydrogen to the reductive end group in sugar. According to the carbon number, hydrogenated sugar is classified into tetritol, pentitol, hexitol and heptitol (4, 5, 6 and 7 carbons, respectively). Among them, hexitol, having 6 carbons, includes sorbitol, mannitol, iditol, galactitol, etc.—in particular, sorbitol and mannitol are very useful materials.
Anhydrosugar alcohol has a dial form with two hydroxyl groups in the molecule, and can be produced by using hexitol derived from starch. Because anhydrosugar alcohol is an environmentally friendly material derived from recyclable natural resources, it has received much interest for a long time and researches on its production have been proceeding. Among such anhydrosugar alcohols, isosorbide produced from sorbitol has the widest industrial applicability at present.
Anhydrosugar alcohol can be used in various fields including treatment of heart and blood vessel diseases, medicaments such as patch adhesive, mouthwash, etc., solvents for compositions in the cosmetics industry, emulsifiers in the food industry, etc. In addition, it can increase the glass transition temperature of polymer materials like PET, polycarbonate, polyurethane, etc., and improve the strength of such materials. Furthermore, because anhydrosugar alcohol is a biodegradable, environmentally friendly material, it is very useful in the plastics industry. It is also known that anhydrosugar alcohol can be used as an adhesive, environmentally friendly plasticizer, biodegradable polymer, and environmentally friendly solvent for water-soluble lacquer.
In particular, because of its diol form with two hydroxyl groups in the molecule, anhydrosugar alcohol is very valuable as a property modifier in the plastics industry. The ion content and color of anhydrosugar alcohol are important factors influencing the physical, chemical and optic properties during polymerization in the plastics industry.
As such, anhydrosugar alcohol is receiving much interest because of its wide applicability. However, there have been few cases of practically using it industrially so far. This is because the conventional methods of producing anhydrosugar alcohol have limitations of high cost for the catalyst used in the dehydration reaction, low conversion rate, and low yields of distillation and purification, etc.
In the case of producing anhydrosugar alcohol by dehydrating hexitol and distilling the converted reaction liquid, depending on the dehydration reaction and the distillation condition, various byproducts as well as the desired anhydrosugar alcohol are generated, and formation of polymer materials also increases (Starch/Starke, vol. 38, pp. 26-30). In addition, the color becomes dark due to carbonization of hexitol or formation of another coloring substance(s), characteristics of which are not clearly known, and thus decolorization becomes difficult.
Accordingly, purification treatment for the dehydrated and distilled anhydrosugar alcohol is essential, but thereby the overall process becomes complicated, production cost increases, and the yield decreases due to the loss in the purification step.
The required purities of anhydrosugar alcohol are different according to the use thereof. In the case of use for foods or drugs, impurities harmful to the human body should not exist. In the case of use for polymers requiring optical transparency, the impurities which cause color formation or colorization during the synthesis and processing should not be contained. In addition, the impurities which undesirably increase or decrease the polymerization degree or polymerization rate during polymer synthesis should not be contained, either. Even if the impurity content is allowable for foods and drugs, it may not be allowed in polymer synthesis since the color formation or the change of polymerization property may be caused.
In particular, when anhydrosugar alcohol is used in the process of synthesizing plastics, etc., high ion content in anhydrosugar alcohol makes control of the polymerization rate difficult. Thus, ion content in anhydrosugar alcohol is an important factor for the application of anhydrosugar alcohol, but an efficient purification method for reducing it is hard to find.
Several methods of purifying anhydrosugar alcohol are generally known. One of them is a method of crystalizing anhydrosugar alcohol distillate by using an organic solvent. However, in the case of using anhydrosugar alcohol purified by this method in polymer synthesis, if the organic solvent is removed insufficiently, the remaining organic solvent may cause a change in the polymerization property. In addition, this method is not preferable since it has problems of insufficient removal of color-forming substances, requiring extra costs for facility and operation for crystallization, environmental harmfulness and unstable handling due to the use of organic solvent, etc. As an alternative, a method of crystallization using water as a solvent has been known, but there are problems of low efficiency of impurity removal and serious lowering of yield due to high hydrophilicity of anhydrosugar alcohol.
A method of purifying anhydrosugar alcohol distillate by using ion exchange resin and active carbon is also known. For instance, Korean Patent No. 10-0772255 discloses a method of treating anhydrosugar alcohol distillate with an adsorbent such as active carbon, etc., subsequently treating it with ion exchange resin, and further treating it with an adsorbent such as active carbon, etc. again. However, the purification method disclosed in this patent requires multiple processes of adsorption-ion exchange-adsorption, and even though purified as such, the final anhydrosugar alcohol has a relatively high conductivity of about 20 μs/cm or less. Furthermore, this patent makes no mention about the color property of the finally purified anhydrosugar alcohol.
Therefore, a technology of purifying anhydrosugar alcohol, which can remarkably reduce the ion content and the conductivity and improve the color property while reducing cost through more simplified process, is still required.